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The game is broken into stages; each stage is a single boss battle, resulting in a constant stream of bosses. To progress to the next stage, the boss must be completely destroyed within the time limit. The game starts with a default time limit of 180 seconds, with 30 seconds rewarded for every boss defeated and 20 seconds lost for each time the player is destroyed. The player flies a relatively small green ship. There are no power-ups - the player's only weapon is a very rapid-fire revolving cannon which is fired by holding down the shoot button (Z key by default). By default, it fires straight ahead with a small spread. Alternatively, after the switch button (D key by default) is pressed, a green arc is displayed originating from the ship and widening as distance increases. This arc represents the spread of the weapon fire and does not change while the weapon is firing. It can, however, be modified based on the ship's motion while not firing. The arc will rotate slowly to face the direction opposite the ship's motion. Also, the arc will widen as the player moves toward the enemy and vice versa.

The main gameplay feature is the bosses change from stage to stage. Based on how a boss is destroyed, the next boss will adapt itself to defend against previously used strategies and force the player to change tactics. For example, if the front section of one boss is destroyed the next boss will have increased armor in that area. If the player's ship is hit by a certain kind of weapon, but still defeats the boss, the next boss is likely to have more weapons of that type.[2]

The game does not include music, but provides instructions in the read me file on how to provide one's own music. It also includes sound effects.[3] As is common in manic shooters, the area of the player's ship vulnerable to collisions is much smaller than the ship itself, enabling the ship to squeeze through small gaps.

The initial form of the boss is called the Pure Heart, which will take on new characteristics in response to the player's assault.

The top and bottom halves will gain characteristics based on the order of destruction of parts that are attached to the side of exactly one part, provided that there are available extensions to add for that half. The boss always has a vertical line of symmetry, so any destruction on one side of the boss is considered to have happened on the other side as well for extension purposes.

The Pure Heart has 8 vulcan cannons, and 4 corner pieces. Because the corner pieces are introduced as being attached to multiple parts, their destruction has no bearing on extension placement. However, the guns do affect extension placement, each being attached to a single corner piece. For example, if a player destroys the gun that is lowest on the right, by the symmetry rule the gun on the left that matches is also considered destroyed. The boss will be extended at that point on both sides next round. Destruction of the other gun attached to that corner piece at this time will not cause an extension, because an extension is already being made to that piece. Destruction of the guns on the opposing piece also has no effect because the extension for that piece has already been chosen.

The next form adds a new body piece and two guns to each side of the bottom. If the player destroys one of the original guns left on the bottom before destroying the new body piece, it will cause an extension. However, if the new body piece is destroyed first, then the destruction of the original gun will NOT cause an extension, because another piece was attached to the one that was destroyed first. However, there will be an extension on the new body piece, unless it is somehow destroyed without destroying either of its guns.

Note that if the original gun is destroyed first, and then one attached to the new piece is destroyed, it results in two extensions.

Once a certain number of extensions have been made to a side (top or bottom), that side evolves.

Download.com awarded the game a 5-star editor's rating and described the game as a classic in the making, "Warning Forever will entice nearly every gamer who enjoys a space shootout".[3] The staff of Computer Gaming World, writing for gaming blog 1up.com said "This game is among the most addictive platform shooters in the business".[4]Something Awful's Zack Parsons praised the game for its graphics and gameplay, the only negatives listed were "It never ends" and "Seriously, you will lose your job because of this game".[5] This game is part of the permanent collection of the travelling exhibition Game On.

Battleships Forever is a free real-time strategy game that uses graphics inspired by Warning Forever's boss for its battleships, and has been nominated for IGF awards. It has an active community on the internet, and has been praised for the amount of customization possibilities it offers.